1. Field of the Invention
The invention relates to a system for recording and detecting marking signals which can be stored on and scanned from a record carrier and which can be distinguished from one another, comprising a recording device for recording the marking signals to be stored and a detection device for detecting the marking signals being scanned. The recording device includes means for generating the marking signals to be stored and the detection device includes means for detecting the marking signals being scanned, the marking signal generator having a first stage for generating a first signal at a first frequency, a second stage for generating a second signal at a second frequency, a first time-control stage by means of which the duration of the first signal can be defined, and a second time-control stage by means of which the duration of the second signal can be defined. The first time-control stage defines a predetermined value T for the duration of the first signal and the second time-control stage at the same time defines the value zero for the duration of the second signal when a marking signal of a first type is to be generated consisting only of the first signal. The first time-control stage defines the value T for the duration of the first signal and the second time-control stage at the same time defines the value T for the duration of the second signal when a marking signal of a second type is to be generated consisting of the first signal and the second signal. The detector means includes a first detector stage for detecting a signal which is scanned from the record carrier and which corresponds to the first signal, a second detector stage for detecting a signal which is scanned from the record carrier and which corresponds to the second signal, which stages each supply an output signal when the corresponding scanning signal occurs, and a logic stage which, depending on the output signals of the two detector stages, generates and supplies detection signals corresponding to the marking signals scanned from the record carrier.
2. Description of the Related Art
Such a system is employed in a dictation system manufactured and marketed by N.V. Philips and referred to as the "mini-cassette dictation system", the system comprising pocket dictation machines and portable dictation machines, which enable dictations to be recorded as well as reproduced, and transcription machines, which are only suitable for the reproduction of dictations. In this known dictation system, the system employed therein for recording and detecting marking signals, which is of the type defined in the opening paragraph, enables marking signals of two different types to be recorded on a magnetic tape record carrier to mark specific dictated passages by recording the marking signals and to locate these passages by detection of the marking signals scanned. In this system a marking signal of a first type, which is constituted by a first sinewave signal having a frequency of 40 Hz and a duration, for example, of 650 ms, and which is recorded on the magnetic tape at a given normal speed of transport of said tape, then enables the end of a dictation to be marked. By means of a marking signal of the second type, constituted by the first sinewave signal having a frequency of 40 Hz and a second sinewave signal having a frequency of 1500 Hz, which signals are generated simultaneously for the same length of time of 650 ms and are recorded on the magnetic tape at the same normal speed of transport of this tape, it is possible to mark dictated passages for which the dictator has given or wishes to give special instructions, indications for correction or the like, to be observed during transcription of the dictation. As is evident from the foregoing, the marking signals of the first and the second type in the known system differ from each other in that a marking signal of the first type comprises only one signal having a given frequency and a given duration, and a marking signal of the second type comprises two signals, one signal having the same frequency as the first marking signal and the other signal having a different frequency, which signals each have the same duration as the first marking signal.
In the known system the marking signals of the first and the second type scanned from the tape are detected at a speed of transport of the magnetic tape which is higher than the speed at which the marking signals are recorded, this speed of transport being, for example, a factor of 20 higher than the speed of transport during recording of the marking signals. This higher speed of transport during detection of the marking signals being scanned results in the signals forming the marking signals being subjected to a frequency conversion by a factor of 20, so that during detection of a marking signal of the first type a signal having a frequency of 800 Hz is obtained, which corresponds to the first sinewave signal of a frequency of 40 Hz, and during detection of the marking signal of the second type a signal having a frequency of 800 Hz, corresponding to the first sinewave signal having a frequency of 40 Hz, and a signal having a frequency of 30 kHz, corresponding to the second sinewave signal having a frequency of 1500 Hz, is scanned from the magnetic tape. For this reason the detection device for the marking signals of the first and the second type scanned from the magnetic tape in the known system comprises detector means comprising a first detector stage for detecting a signal of a frequency of 800 Hz and a second detector stage for detecting a signal of a frequency of 30 kHz, both stages each supplying an output signal when the corresponding signal occurs, which output signal is applied to a logic stage following the two detector stages. If only the first detector stage supplies such an output signal the logic stage will produce a detection signal corresponding to a marking signal of the first type. If both detector stages supply an output signal the logic stage will supply a detection signal corresponding to a marking signal of the second type. These detection signals can be applied to an optical or acoustic signalling device. However, they may also be applied to a stop device for stopping the drive system for the magnetic tape, so that upon detection of a marking signal the magnetic tape transport is discontinued in that the drive system for the magnetic tape is switched off.
In the prior-art system for recording and detecting marking signals, as described in the foregoing, it is possible only to record and detect two different marking signals of a first and a second type. This is found to be a drawback limiting the use of the apparatuses of the known dictation system because there is a need for further marking possibilities by means of marking signals, which cannot be realised in the known system for recording and detecting only two marking signals.